The present invention relates generally to the production of high density, dead burned magnesia (MgO). More in particular, it relates to the production of dead burned magnesia directly from an aqueous magnesium hydroxide slurry in a single burning step.
It is well known that dead burned refractory magnesia can be prepared from an aqueous magnesium hydroxide slurry by heating the slurry to achieve decomposition of magnesium hydroxide to magnesium oxide, followed by the densification of the magnesium oxide to the refractory grain material.
One conventional method of producing dead burned refractory magnesia is known as the "double burning" or "two-stage" process. This process involves: (1) heating (calcining) the magnesium hydroxide slurry at a temperature of from about 900.degree. C. to about 1200.degree. C. to produce a chemically reactive oxide of magnesia; (2) compacting the thus calcined magnesia in a high pressure briquetting roller; and (3) dead burning (sintering) the briquettes in a rotary or shaft kiln at a temperature of from about 1500.degree. C. to about 2000.degree. C. A general description of a "double burning" process of producing dead burned refractory magnesia is described in greater detail in U.S. Pat. No. 3,060,000.
Although the "double burning" process can produce dead burned refractory magnesia of acceptable quality, the process suffers from a number of disadvantages. Among the disadvantages are: excessive dust formation during the briquetting and calcining steps; high wear of the briquetting machine due to the abrasive nature of the calcined magnesia; fragility of the magnesia briquettes which necessitates special care in handling equipment during the sintering process; and the necessity of providing two high energy burning operations, i.e., calcining and sintering.
Another known process for the production of dead burned refractory magnesia is described in U.S. Pat. No. 4,033,778. This process involves the dewatering and compaction of magnesium hydroxide sludge by the application of pressure. The magnesium hydroxide filter cake obtained by pressure filtration can be directly dead burned to refractory magnesia. This process has the disadvantage of yielding granular dead burned magnesia having a density of only about 3.2 to about 3.35 grams per cubic centimeter (g/cc), depending upon the impurities in the initial starting material.
Another process for producing dead burned refractory magnesia is described in U.S. Pat. No. 3,378,615 and in the publication "Development of New Process for Sea Water Magnesia Production", by R. T. Randle, H. Heasman, and W. C. Gilpin, Refractories Journal, pages 13-17. This process involves first removing some of the free water content from an aqueous magnesium hydroxide solution and then folding, mixing, and kneading the resulting paste. This process results in a magnesium hydroxide cake containing less than 75 percent solids which may be dead burned to a refractory product. However, the process described in U.S. Pat. No. 3,378,615 is unsatisfactory because unless the free water content of the starting material is controlled, prior to the kneading operation, within a well defined range, the subsequent firing operation produces a large amount of undesirable dust. Moreover, the Refractories Journal article reports that when the percent solids in the magnesium hydroxide cake exceeds 75 percent by weight, the cake crumbles when kneaded and both the grading and the density of the fired cake rapidly decrease.
It would be desirable, therefore, to produce a dead burned refractory magnesia of high density in a single burning operation without the necessity of a calcining step. Ideally, this method should produce magnesia having a density at least as great as, and preferably substantially greater than, the conventional processes.